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Periodic Table of the Elements 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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Periodic Table of the Elements 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 PERIODIC TABLE OF THE ELEMENTS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14-1 Chapters 14 and 23. PROPERTIES OF THE ELEMENTS First, let’s look at Periodic Table and Group numbering. 14.1 Hydrogen. The simplest atom, ~90% of all atoms in the universe. The only element whose isotopes are each given a different symbol and name. 1H (or H, protium) = one proton (p+), plus one e- surrounding it. 2H (or D, deuterium) = one p+ and one neutron (n), plus one e- 3H (or T, tritium) = one p+ and two n, plus one e-. Deuterium (2H) was produced in the ‘Big Bang’ — it is too fragile to survive fusion conditions in the stars (which produce the lighter elements) or supernovas (which produce heavier elements). Hydrogen is the exception in the periodic table — it cannot be satisfactorily classified in any group: it has similarities both to (a) group 1 metals such as Li, Na, etc, in forming H+ and (b) group 17 non-metals such as F, Cl, etc, in being H2 (H-H with a single covalent bond) in its stable - elemental form (compare F2, Cl2, etc) and also forming H (hydride ion) analogous to F-, Cl-, etc. 14-2 Brief Summary of Hydrogen Chemistry - most commonly forms covalent compounds; ionic compounds are rare. - high ionization energy (unlike group 1, because e- close to nucleus without other e’s to shield it) and low electronegativity (unlike group 1, because only one proton to attract e-s). unlike groups 1 and 17 in that H+ and H- ions are rare (whereas Na+, K+, etc, ions are common, as are F-, Cl-, etc ions) because they usually bond + - + covalently to other things e.g., H3O , OH , NH4 , etc Very rare exceptions are certain ionic salts of H-, the hydride ion, in compounds such as NaH and CaH2 (similar to NaCl and CaCl2). Ionic Hydrides (H-). With very strong reducing agents (Na(s), Ca(s), Li(s), etc.), hydrogen is reduced to H- = ionic hydrides. e.g. 2 Na (s) + H2 (g) 2 NaH (s) - - Note that: H2 (g) + 2 e 2 H (g) Eº = -2.23 V (very negative!) Hydrides are thus very reactive (strong reducing agents) and will either: + (1) react with a H and go to H2 (g): + - NaH (s) + H2O (l) Na (aq) + OH (aq) + H2 (g) or (2) reduce something and go to H2 (g): TiCl4 (l) + 4 LiH (s) Ti (s) + 4 LiCl (s) + 2 H2 (g) 14-3 Covalent Hydrogen Compounds. Very common and stable: CH4,NH3,H2O, HF, etc, etc. These other elements have higher electronegativity than H (H = 2.2, C = 2.5, N = 3.1, O = 3.5 F = 4.1) we think of these as containing H+ oxidation state and C4-,N3-,O2-,F- oxidation states. e.g. F2 (g) + H2 (g) → 2HF (g) H2 is a very important gas, for many reasons. For example: N2 (g) + 3 H2 (g) 2NH3 (g) G < 0 (spontaneous) but very slow under normal conditions due to very strong NN reaction run in industry at high T (~400 ºC) and pressure (250 atm) with an Fe catalyst to speed it up. This is called the Haber process,andisthemainsourceofNH3 for the fertilizer industry. Metallic (Interstitial) Hydrides. H2 molecules and H atoms can occupy space in-between the atoms of a metal. In particular, palladium (Pd) has a high affinity and can hold vast amounts (Pd: 935 times its volume = PdH0.5). Best thought of as a solution of the gas in the metal! Came to people’s attention during Cold Fusion stories of late 1980’s. Formation of Pd/H2 is used to purify H2 from gas mixtures. 14-4 Figure 14.2 A metallic (interstitial) hydride Many transition metals form metallic (interstitial) hydrides, in which H2 molecules and H atoms occupy the holes in the crystal structure of the metal. 14-5 PERIODIC TRENDS ACROSS THE PERIODIC TABLE Increasing: metallic character non-metallic character ionic oxides covalent oxides basicity of oxides acidity of oxides electropositivity electronegativity ALSO: (i) Trends for a given element are affected by changes in its oxidation state (ii) For Periods 2 and 3, atom size decreases left-to-right Figure 14.1 Examples: Which is the most metallic element a) Li or Be b) Be or Mg c) Al or K Which is the most acidic oxide? (i.e. dissolve in H2O to give acidic soln) a) Na2OorCl2O7 b) BaO or As2O5 Which is the stronger reducing agent? a) Al or Mg b) Na or K c) Br- or Cl- Which is the stronger oxidizing agent? a) S or Se b) Br2 or Cl2 c) O2 or F2 Summary: The more metallic an element, the more basic is its oxide, the more ionic is its oxide, and the more electropositive (less electronegative) it is. 14-6 Table 14.1 Trends in Atomic, Physical, and Chemical Properties of the Period 2 Elements. Trends in atomic radius, ionization energy, and electronegativity across Period 2. 14-7 GROUP 1. ALKALI METALS (ns1) Found as M+ in minerals (loss of ns1 electron). Too reactive (strong reducing agents) to be found as the free metal. Prepared in industry by electrolysis of melted salts. All well-studied except Francium — radioactive, longest-lived isotope is 283Fr: half-life of only 21.8 min. Estimated only ~25 g on Earth at any one time Properties. Soft, silvery metals at 20 ºC. All react vigorously with water to give H2 gas: the reactivity increases down group. M (s) + H2O (l) H2 (g) + MOH (aq) (video) half-reactions: M (s) M+ (aq) + e- - - H2O (l) + e ½H2 (g) + OH (aq) + - overall: M (s) + H2O (l) ½H2 (g) + M (aq) + OH (aq) 14-8 All react with O2 gas but products depend on metal: 2- Li gives Li2O (contains O i.e. normal oxide ion) 2- 2- Na gives Na2O2 (contains O2 =O-O i.e. peroxide ion) - - K, Rb, Cs give MO2 (contains O2 =O-O i.e. superoxide ion) All M(s) must therefore be stored under inert oils to prevent reactions with air and water. Note: Burning in air (rather than pure O2 gas) gives above products, but Li (and only Li) also reacts with N2 (g) to give the nitride, Li3N, containing Li+ and N3-. 6 Li (s) + N2 (g) 2 Li3N 14-9 Important Question – WHY is Li different? The difference between Li and the rest + of group 1 (e.g. gives oxide with O2,reactswithN2, etc) is due to the small size of Li -leadstohigh charge-to-size ratio. This also causes Li compounds to have significant covalent character (i.e. LiCl is still fairly ionic but has noticeable covalent character, whereasNaClisveryionic).Li+ has too high a charge-to-size ratio (“charge density”) tobehappyascompletelyfreeLi+ ion, so it shares electrons a bit with anions (i.e. some covalency). *** For this and other reasons (see later), the top member of every group is significantly different from the rest *** Note 1: The decreasing charge-to-size ratio down the group (i.e. Li+ >Na+ >K+ >Rb+ > + Cs ) explains the products on reaction with O2. The highest charge density (highest charge-to-size ratio) is Li+, and this forms the highest charge density O2- salt, 2- - whereas down the group the metals form the O2 (peroxide) and then the O2 (superoxide) salts, i.e. decreasing charge density ions. i.e., high charge density Li+ favors forming a salt with high charge density O2-,but down the group progressively lower charge density M+ favor formation of progressively lower charge density anions. Note 2: Group 1 metals have very high 2nd Ionization Potential -means M2+ not possible to make (in stable form at 25 ºC). Of course, in places such as the surface of the sun, all sorts of other ions are possible. 14-10 GROUP 2. ALKALINE EARTH METALS (ns2) Occur in nature as M2+ ― too reactive to exist as free metals. Radium (Ra) radioactive: 226Ra: half-life of 1599 years. Again, Be slightly different from the rest due to high charge-to- size ratio (cf. Li in group 1): very hard metal and toxic. Others softer and not toxic. Also, Be compounds are mostly covalent, since Be2+ has too high a charge density to be happy as the free ion in ionic compounds – therefore it shares electrons with other atoms, decreasing its charge density (i.e. covalent bonding). Be is unreactive to H2O; Mg reacts slowly with cold H2O, but fast with steam; others react vigorously with H2O. M (s) + 2 H2O (l) M(OH)2 (aq) + H2 (g) All react with O2 to give oxides MO except Ba, gives peroxide BaO2. Mg reacts with N2 gas to ionic nitride, like Li. Others only at high T. 3Mg(s)+N2(g) → Mg3N2(s) (Mg + CO2) 1st and 2nd ionizations are both very easy M2+ (no M+). 3rd ionization very difficult ― no M3+ 14-11 Diagonal Relationships - similarities in properties between a period (row) 2 element and period 3 element, one group to the right. Consider: Li vs Mg - similar size of Li+ vs Mg2+ (0.76 vs 0.72 Å) and similar properties (e.g., both give nitrides with N2, salts with similar solubilities and thermal stabilities, and significant covalency in their bonds). The origin of these similarities is again the high charge-to-size ratios of Li+ and Mg2+.MgisbiggerthanLi,soMg2+ and Li+ end up being similar size. 14-12 Section 14.5. GROUP 13. (ns2np1) Metallic character decreases moving right, and we find that boron (B) is not a metal = “metalloid” or “semi-metal”.
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